1. Field of the Invention
The present invention relates to a fusing apparatus used in an electrophotographic system, an image forming apparatus provided with the same, and a heating apparatus having a sheet heating element.
2. Description of the Related Art
An image forming apparatus using an electrophotographic process (hereinafter merely referred to as an “image forming apparatus”) includes a photoconductor, a charging unit, an exposure unit, a developing unit, a transfer unit, and a fusing unit, for example. The image forming apparatus performs a charging process, an exposure process, a developing process, a transfer process, and a fusing process with the use of the photoconductor and these units, whereby forming an image onto a sheet-type recording medium (hereinafter merely referred to as a “sheet”).
A fusing apparatus of a thermal roller fusing system is used, for example, as the fusing unit that performs the fusing process. The fusing apparatus of the thermal roller fusing system includes a fuser roller and a pressure roller. The fuser roller and the pressure roller are a pair of rollers that are in pressed contact with each other. At least either one of the fuser roller and the pressure roller includes a heat source serving as a heating unit, such as a halogen heater, mounted therein.
In the fusing process, the heat source heats the roller pair to a predetermined temperature required for the fusing (hereinafter referred to as a “fusing temperature”). Thereafter, the recording medium having a non-fused toner image formed thereon is conveyed to a fuser nip portion that is a press-contact portion between the fuser roller and the pressure roller. The non-fused toner image is fused onto the recording medium such as a sheet due to the heat transmitted from at least either one of the fuser roller and the pressure roller and the pressure from the fuser roller and the pressure roller, when it passes through the fuser nip portion. Although the temperature of the portion of the nip portion where the recording medium passes (hereinafter referred to as a “sheet passing portion”) is decreased, the temperature of the sheet passing portion then rises to the fusing temperature, since the heat is supplied from the heat source.
A fusing apparatus provided to an image forming apparatus capable of performing a full-color printing uses a fuser roller provided with an elastic layer made of a silicon rubber on its surface (hereinafter referred to as an “elastic roller”). For forming a full-color image in which a plural colors of toners are used, more toner needs to be fused compared to the case of a monochrome image. When the elastic roller is used, the surface of the elastic roller is elastically deformed corresponding to irregularities on the non-fused toner image at the fuser nip portion. Specifically, the elastic roller and the non-fused toner image are brought into contact with each other as if the elastic roller covers the non-fused toner image. Therefore, a fusibility for fusing the full-color image, which uses a lot of toner, can be enhanced. A releasing performance of a color toner, which is liable to be offset compared to a monochrome image, can be enhanced due to an effect of releasing a distortion of the elastic layer on the surface of the elastic roller. Specifically, on the elastic layer that is compressed and deformed at the fuser nip portion, the deformation is released at an exit of the fuser nip portion. Therefore, a deviation is generated between the elastic layer and the toner image at the exit of the fuser nip portion. As a result, an adhesion force of the elastic layer to the toner image is reduced, resulting in that the releasing performance is enhanced. Since the elastic layer is deformed to have a concave shape due to the press-contact between the fuser roller and the pressure roller at the fuser nip portion, the shape (nip shape) of the path through which the recording medium passes becomes convex toward the fuser roller. Therefore, the curvature of the surface of the fuser roller at the portion where the recording medium is separated from the fuser roller increases, resulting in that a separation property of the recording medium can be enhanced. As a result, a structure of being capable of separating the recording medium from the fuser roller (self-stripping structure) can be realized without providing an auxiliary separating unit, e.g., a separating claw, for separating the recording medium from the fuser roller. This structure can avoid the formation of a defective image caused by the separating unit.
In order to respond to an increased speed in an image formation, the width of the fuser nip portion (hereinafter referred to as “fuser nip width”) is necessarily widened. There are two methods, which are a method of increasing the thickness of the elastic layer of the elastic roller, and a method of increasing the diameter of the elastic roller, as a method of increasing the fuser nip width. However, a thermal conductivity of the elastic layer of the elastic roller is extremely low. When the thickness of the elastic layer of the elastic roller increases in a structure in which the heating unit is provided in the elastic roller as in the conventional case, a thermal conductivity from the inside of the elastic roller to the surface thereof is deteriorated, which might increase a warm-up time. When a process speed involved with the image formation is increased, a peripheral speed of the fuser roller has to be increased corresponding to the increased process speed. However, the recovery of the temperature of the fuser roller, which is reduced at the sheet passing portion, is too late, which entails a problem that the temperature of the fuser roller cannot follow the fusing temperature. When the diameter of the elastic roller is increased in order to secure the time for the temperature recovery, a power consumption of the heating unit might increase.
A fusing apparatus of a belt fusing system is described in Pamphlet of International Publication No. WO99/00713 in order to solve the foregoing problems. The belt fusing system includes a fuser roller, a pressure roller, a heat roller, and an endless belt. The endless belt is stretched between the heat roller having a heater provided therein and the fuser roller, wherein the fuser roller and the pressure roller are in contact with each other via the endless belt. In the belt fusing system, the heat roller serving as the heating unit heats the endless belt having a small heat capacity, whereby a warm-up time can be shortened, compared to the structure in which the elastic layer having a large heat capacity is heated. Since the heating unit does not have to be incorporated in the fuser roller, the deterioration in the thermal conductivity does not become a problem, even if the elastic layer made of a sponge rubber, for example, having a low hardness is formed to be thick, resulting in that the wide fuser nip portion can be secured.
Japanese Unexamined Patent Publication No. 7-201455 proposes a fuser of a film heating system. In the film heating system, a sheet heating element is used as the heating unit, wherein a heat from the heating unit is applied to a recording material, serving as a material to be heated, through a film. A non-fused image on the recording material is fused by the heat as a permanent image.
In the fusing apparatus of the film heating system, the heat capacity of the sheet heating element is smaller than the heat capacity of a conventional halogen lamp heater. Specifically, the heat capacity of the heating unit can be reduced more than in the conventional case, whereby power saving can be attained, and the warm-up time can be shortened.
The present inventors have developed a fusing apparatus using a fuser belt of an endless belt type. A fuser apparatus according to the present invention includes a fuser belt that is stretched between a fuser roller and a tension roller to form a loop-type moving path, wherein a heating member is brought into contact with an inner side of the stretched fuser belt so as to heat the fuser belt, as illustrated in a later-described FIG. 3. The fuser belt is in contact with a pressure roller at a press-contact point between the fuser roller and the pressure roller opposite to the fuser roller. A recording medium having a toner image transferred thereon is guided to the press-contact point between the pressure roller and the fuser belt, wherein the toner image is heated and fused by the fuser belt to be fixed onto the recording medium.
The heating member includes, in this order from the fuser belt, a heat-conductor, a sheet heating element, a heat insulating member, a pressure member, and a reinforcing member.
As illustrated in a later-described FIGS. 5A and 5B, the sheet heating element includes plural resistance heating layers that are formed on a surface of a substrate which has a slender shape in a widthwise direction so as to extend in a longitudinal direction of the substrate. One end and the other end of each of the resistance heating layers are connected by end electrodes. A conduction portion extending in the widthwise direction of the substrate is formed at plural places at a middle portion sandwiched between the end electrodes at both ends, wherein the different resistance heating layers are connected by the conduction portion. The heat-conductor is made of a material having excellent thermal conductivity such as an aluminum that is a rigid body. The pressure member presses the sheet heating element against the heat-conductor so as to efficiently transfer a heat from the sheet heating element to the fuser belt.
In the fusing apparatus having the above-mentioned configuration, the present inventors have found problems described below involved with a setting of a pressure position where the sheet heating element is pressed against the heat-conductor.
(1) Pressure Position at Central Portion in Longitudinal Direction of Substrate
In the present invention, the conduction portion is formed at plural positions in the longitudinal direction for connecting in the widthwise direction of the substrate the different resistance heating layers, in order to average a variation in the resistance of the respective parallel resistance heating layers in the longitudinal direction of the substrate. The resistance heating layers are divided into several blocks in the longitudinal direction thereof by forming the conduction portion, wherein the respective blocks are electrically connected in series. This is to average a variation in a unit of the block so as to reduce a temperature unevenness, even if there is the variation in the resistance value of the parallel resistance heating layers composing each block.
When the pressure position is close to the conduction portion in this case, the substrate thermally expands at the adjacent conduction portions to warp, with the result that a gap is formed between the sheet heating element and the heat-conductor. When the gap is locally formed in the longitudinal direction of the substrate, a transfer of the heat at this portion is hindered, resulting in that the temperature of the sheet heating element is locally increased. If so, this portion further expands thermally, which causes a significant warp. In an extreme case, the substrate is broken, or the resistance heating layer is fused by the heat, so that a uniform heat generation cannot be attained. Although the substrate is not broken or the substrate does not become defective, a problem arises in which a temperature unevenness is caused in the longitudinal direction of the substrate.
(2) Pressure Position at End in Longitudinal Direction of Substrate
As for the pressure position in the vicinity of the end of the sheet heating element, when the pressure position is set within a fixed range closer to the center than to the end of the resistance heating layer, the sheet heating element thermally expands to warp, resulting in that a gap is formed between the heat-conductor and the sheet heating element. As a result, the substrate is broken, or the resistance heating layer is locally fused, as in the case of (1). Although the substrate is not broken or the substrate does not become defective, a problem arises in which a temperature unevenness is caused in the longitudinal direction of the substrate.
These problems are unique to the present structure in which the sheet heating element is pressed against the rigid member. Specifically, Pamphlet of International Publication No. WO99/00713 and the present invention are different from each other in that the sheet heating element is pressed against the elastic member such as the pressure roller or the rigid member such as a member made of an aluminum. According to the configuration of the present invention, there is no chance that the sheet heating element is pressed against the elastic member having large heat capacity, whereby the configuration is advantageous from a viewpoint of power saving and shortening the warm-up time. On the contrary, Pamphlet of International Publication No. WO99/00713 describes the configuration in which the sheet heating element is pressed against the pressure roller, which is the elastic member, via the fuser belt, so that this configuration is disadvantageous in power saving and shortening the warm-up time. However, the deformation of the sheet heating element due to the warp is absorbed by the pressure roller, which is the elastic member, so that a gap is difficult to be formed. Therefore, the gap is not formed between the sheet heating element and the pressure roller, which means the problem in the present invention involved with the pressure position hardly arises.